Non-dense sintered ceramic molded body having at least two layers

ABSTRACT

A non-dense sintered ceramic molded body having at least two layers, wherein a first powdery ceramic material forming a layer is contacted with at least a second powdery material forming at least a second layer. The body has a color gradient and maintains dimensional stability during sintering and forming. An admixing component and a common sintering temperature are used to control the volume decrease of the layers during sintering.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of copending U.S. patentapplication Ser. No. 14/391,042 to Dorn et al., entitled “Process forProducing a Non-Dense Sintered Ceramic Molded Body Having at Least TwoLayers” which is a national stage filing of PCT Application No.PCT/EP2013/057917, which claims priority to European Patent ApplicationNo. 12164282.1 filed Apr. 16, 2012 and to U.S. Provisional ApplicationNo. 61/644,055 filed May 8, 2012, each of which are incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to a process for producing a non-densesintered ceramic molded body having at least two layers and to a moldedbody obtainable by the process according to the invention.

INTRODUCTION AND SUMMARY OF THE INVENTION

The use of glass-ceramic restorations has proven effective for therestoration of dental defects, especially in the region of the mouth.Because of the low strength of such restorations as compared to thosemade of stabilized zirconia, these materials, which meet aestheticrequirements, are not indicated for the preparation of bridges or forrestoring patients suffering from masticatory disorders (bruxism). Forthe preparation of posterior bridges or crowns, it has been usual toproduce scaffolds of a high strength material, which subsequently needto be faced with a glass ceramic layer. If fully anatomical crowns orminimally reduced crowns are to be produced by CAD/CAM technologies, theblanks obtained are only glazed or provided with a thin veneer layer.This is an alternative to a cast crown and is not to be recognized as arestoration if possible. However, this requires that the crown is notunicolored, but has a multicolored design, a color gradient running fromthe tooth neck towards the incisal edge/occlusal surface from dark tolight in order to reproduce the color gradient of a natural tooth asexactly as possible.

Articles made of zirconia, especially for dental purposes, have becomeestablished because of their comparatively high bending strength.However, the common zirconia materials for dental purposes areunicolored and can be immediately recognized as a restoration because oftheir light color. Therefore, copings for crowning prepared therefrommust always be faced in order that the restoration fits aestheticallyinto the remaining teeth. In order to achieve a natural coloring, porousmonolithic zirconia restorations, which may also have been precolored ina single color, are matched with coloring liquids to the individualsituation in the patient.

EP 1 859 757 A discloses ZrO₂-based compositions as well as unicoloredand multicolored blanks made of oxide ceramics, and a process for thepreparation thereof, in which a) oxide-ceramic powders are coated with acoloring substance, b) the coated powders are preferably classified, andat least one colored powder is filled into a compression mold, c) thecolored powder or powders is/are compressed into a molded body, and d)the compressed molded body is sintered into a blank, and the use of suchblanks for preparing dental restorations.

EP 1 859 758 A discloses a block body of ceramic compositions,especially feldspar-based dental compositions, consisting of at leastone ceramic composition with predefined first optical properties and atleast one second ceramic composition with predefined second opticalproperties, and a transition zone between the two ceramic compositionsconstituted of varying blends of said at least two ceramic compositions,the variation gradient of the blends being essentially constant.

EP 1 900 341 A discloses a multicolored molded body, especially made offeldspar ceramics, with superimposed layers for preparing dentalprostheses, at least two successive layers of different colors and atleast two intermediate layers of different colors between at least twosuccessive primary layers of different colors, wherein between theseintermediate layers a change of the color takes place along a directionthat is opposite to the direction of the change in color between theprimary layers.

WO 2007/137696A discloses green body consisting of at least twodifferent molding powder mixtures each containing a ceramic powder, anda coloring metal compound and/or a coloring pigment which are compressedto form the green body. The green body ceramics are particularlysuitable as color-graded dental ceramics (blanks), which serve asall-ceramic restorations. The powder mixtures can consist out of azirconia granulate and a zirconium silicate material, in which thezirconium silicate changes the chemical structure and lowers themechanical properties (e.g. 3-point bending strength).

Iron oxides, for example, are used for coloring zirconia granules. Theseor similar additives influence the sintering behavior because theychange the sintering activity. Thus, a more intensely colored region candiffer from a less intensely colored region in terms of shrinkage ordensity at the same temperature. This deviating sintering behaviorunavoidably provokes deformation of the original geometry, if thesintering process is stopped before the theoretical density is reached,in favor of improved processability.

Mixing pigmented and non-pigmented zirconia granules basically enablesthe preparation of a color-layered geometry or the production of athree-dimensional gradient. For processing by CAD/CAM technology, it isnecessary that such blocks or disks are presintered at a particulartemperature in order to ensure good grindability. The presinteringessentially corresponds to an intermediate stage on the way to the finaldense sintering, as close as possible to the theoretical density.However, the presintering of multicolored blocks is problematic becausea sintering distortion between lighter and darker regions occurs in thementioned intermediate stage. This distortion is caused by differentsintering activities, which are primarily due to the proportion ofcoloring components. However, since the final sintering density of thelayers as achieved in dense sintering is almost identical, the groundobject would again become distorted from the intermediate stage to thefinal stage.

To date, no solutions have been known that enable the production of acolor gradient in zirconia blanks in such a way that they can besubsequently processed in a porous sintered state.

Indeed, it would be basically possible to circumvent the difficulties bydetermining the sintering distortion in advance and calculating acorrection during the processing by CAM software. However, this methodwould reach the limits of travel paths of the CAM machines when thereare undercuts, and therefore is not workable in practice. Also, thismethod does not achieve the object of the invention.

It is desirable to adjust colored and colorless powders or granules ofzirconia in such a way that they do not exhibit distortion in theprocessable intermediate stage and in the final density, i.e., afterprocessing and dense sintering.

The object of the invention is achieved by a process for producing anon-dense sintered ceramic molded body having at least two layers,

-   -   wherein a first powdery ceramic material forming a layer is        contacted with at least a second powdery material forming at        least a second layer;    -   said first powdery material has a presintering temperature T₁        that is higher than the presintering temperature T_(S) of said        at least second powdery ceramic material;    -   the course of a curve of shrinkage S₁ of said at least first        powdery ceramic material differs from the course of a curve of        shrinkage S₂ of said at least second powdery material, wherein        curve of shrinkage S₁ is shifted towards higher temperatures as        compared to curve of shrinkage S₂; and    -   the layers are subjected to a common temperature treatment at a        presintering temperature T_(S) that is lower than the        presintering temperature T₁ and at least equal to T₃ to cause        sintering that remains in a stage of sintering that has not        proceeded to the theoretical density; wherein    -   the curve of shrinkage S₁ is modified by admixing at least one        component having a curve of shrinkage S₃ which material is        compatible with said powdery ceramic material into said first        powdery ceramic material, i.e. has a grain size smaller than the        first powdery ceramic material, to equalize the curves of        shrinkage S₁ and S₂ in the region of the presintering        temperature T_(S).        The compatibility of the compensation powder depends on the        grain size of used components. By mixing and pressing these        different powders no pores or inhomogeneities should derange the        sintered or presintered structure.

An advantage of the process according to the invention resides in thefact, among others, that the relevant properties of a known material aremaintained while the optical properties can be controlled in a desireddirection. Thus, for example, it is possible to design the block withdifferently colored layers in almost any number. Further, it is alsopossible to produce a multicolored block by continuous mixing methodsand by using just a few starting colors, wherein the color grading mayalso be a continuous gradient.

The process according to the invention enables the coloring ofyttrium-stabilized zirconium layers, which can be arranged assuperimposed blocks. Thus, the user gets the possibility to use forrestorations made of zirconia respectively the same starting materialthat has already been largely used for unicolored dental prostheses ordental replacement scaffolds. The physical properties remain essentiallyunchanged as compared to the established product. Disturbing ortime-consuming operations, such as the coloring of the scaffolds or thefacing of the scaffolds, can be reduced to a minimum or are superfluous.Color variations in the restoration to be produced can be defined by asimulation with the software designated for the processing. After theCAD/CAM production off the restoration, the sintering process performedto dense sintering, a glaze layer can be applied if desired.

In one embodiment of the process according to the invention, the firstpowdery ceramic material contains yttrium-stabilized zirconia, which iscommercially available, for example, as granules from the company TOSOH,Japan, under the designation TZ-3YSB-C(Curve S₁, T₁).

In a further embodiment of the process according to the invention, theat least second powdery ceramic material contains yttrium-stabilizedzirconia that exhibits a more intense coloring than that of the firstpowdery ceramic material. The at least second powdery ceramic materialis commercially available, for example, as granules from the companyTOSOH, Japan, under the designation TZ-Yellow-SB-C. (Curve S₂, T_(S))

In another embodiment of the process according to the invention, thecomponent admixed into the first powdery ceramic material is, forexample, the granules obtainable from the company TOSOH, Japan, underthe designation TZ-PX-242A or Zpex (Curve S₃, T₃).

In particular, the presintering temperature T₁ according to the presentinvention is within a range of from 900° C. to 1300° C., especially from950° C. to 1150° C., from 1000° C. to 1100° C., or from 1040° C. to1080° C., T_(S) is within a range of from 850° C. to 1250° C., and/or T₃is within a range of from 800° C. to 1200° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically the shrinking behavior for 3 differentmaterials during sintering (in a representation of three curves ofshrinkage).

FIG. 2A schematically shows a first embodiment of a three-layer blockaccording to the invention;

FIG. 2B schematically shows a second embodiment of a three-layer blockaccording to the invention;

FIG. 2C schematically shows a second embodiment of a three-layer blockaccording to the invention; and

FIG. 2D schematically shows a second embodiment of a three-layer blockaccording to the invention.

DETAILED DESCRIPTION

The invention is explained in more detail in the following. FIG. 1schematically shows the course of the curves S₁, S₂ during sintering ofthe first and second powdery ceramic materials. The presinteringtemperatures of the first and second powdery ceramic materials aredesignated as T₁ and T₂. “Presintering temperature” means thetemperature that is lower than the corresponding sintering temperatureof the respective material at which dense sintering to the theoreticaldensity or at least to very close to the theoretical density occurs. Thecurve of shrinkage of the component compatible with the first powderyceramic material is designated as S₁ in FIG. 1. T_(S) is thepresintering temperature at which the layers forming the molded body aresubjected to the common temperature treatment to form a porous sinteredmolded body.

For example, the first powdery ceramic material is white or only weaklycolored, and the second powdery ceramic material represents a moreintensely couloured powdery ceramic material having a grain size in therange of the first powdery ceramic material. The third curve S₃ has asmaller grain size than the first powdery ceramic material. The curvesof shrinkage S₁ and S₂ of the two powdery ceramic ground materials areso much different that the disadvantageous distortions of the first andsecond layers occur.

Surprisingly, admixing a component with a curve of shrinkage S₃compatible with the first powdery ceramic material to the first powderyceramic material can equalize its curve of shrinkage S₁ to the curve ofshrinkage S₂ of the second powdery ceramic material within the range ofthe selected presintering temperature, or even achieve identical curvesof shrinkage. The shifting of the curve of shrinkage S₁ depends on thelevel of admixing the component compatible with the first powderyceramic material. The compatibility of the component with curve S₃ isrelated with its smaller grain size compared with the first powderyceramic material. In particular the difference in grain size of the twomaterials is at least 5%, more particularly 10% or 20% or even 30% ormore such as 40%.

Said component compatible with the first powdery ceramic material maybe, for example, a zirconia material containing, in addition to yttriain the percent range, especially 6-7%: less than one percent of alumina,traces of silica, and iron oxide as a pigment, wherein less alumina ispresent as compared to the first powdery ceramic material. The particlesize of the component is smaller than that of the first powdery ceramicmaterial, and typically the size of the particles of the component isabout half the size of the particles of the first powdery ceramicmaterial, whose particle size is within a range of 80-100 nm. The BETvalue of the component to be admixed is about double the BET value ofthe first powdery ceramic material. Typical BET values for the componentto be admixed are 11-15 m²/g. Such materials are commercially availableand are distributed under the designation TZ-PX-242A or Zpex from TOSOH,Japan, for the component to be admixed, and under the designationTZ-3YSB-C, TOSOH, Japan, for the first powdery ceramic material. Thematerial forming component 2 is the at least second powdery ceramicmaterial according to the present invention, being provided with ahigher pigmentation, typically in the form of iron oxide, as compared tothe first powdery ceramic material. Iron oxide has a yellowish color.

The following table shows properties of a special first and secondpowdery ceramic material.

TZ-PX-242A TZ-3YSB-C Crystallite Size [nm] 32 36 Particle size D50 [μm]0.43 0.60 Granule Size [μm] 52 60 Bulk Density [g/cm³] 1.06 1.20Specific surface area [m²/g] ca. 12 ca. 6

The component compatible with the first powdery ceramic material isadmixed with the first powdery ceramic material in proportions within arange of 25 percent by weight to 50 percent by weight (especially from30 to 40% by weight) in order to achieve the effect of equalizing thecurves of shrinkage as desired according to the invention.

The molded bodies according to the invention, which can be prepared byapplying the process according to the invention, advantageously show nodistortions that are due to sintering distortion after completion of thepresintering, so that the molded body in this porous sintered state canbe processed by material removal, and does not experience anydeformation after dense sintering.

In a three-layer stacking in FIGS. 2A-2D, the molded body according tothe invention is schematically represented together with informationabout the layer composition and the color values of the individuallayers according to the CIE-Lab color system. Component 1 according toFIG. 2A is a powdery ceramic material forming the first layer 1 andbeing a mixture consisting of white granules, for example TZ-3YSB-C andTZ-PX-242A, commercially available from TOSOH, Japan. The middle layer 2is formed from a mixture of component 1 and a component 2. Component 2is more intensely yellow/brown pigmented granules, for example,TZ-Yellow-SBC. In the embodiment according to FIG. 2A, layer 2 consistsof a mixture of 80% component 1 and 20% component 2. The middle layer 3is formed from a mixture of components 1 and 2 in a proportion of60%/40%. The further embodiments shown in FIGS. 2B-2D are eachcharacterized by a more intense pigmentation.

The molded body according to the invention is advantageously prepared byintimately mixing the first powdery ceramic material with the componentcompatible with this material. The mixed material is filled into a mold,the forming layer is smoothed and then covered with a second layer of amixture of component 1 and component 2. This step is repeated to formthe third layer. The material is compacted under pressure (typically 100to 200 MPa) and then released from the mold.

This is followed by a sintering process in which the temperature israised. The temperature raise is stopped at a temperature selected insuch a way that the molded body obtained can be readily machined, andsubjected to dense sintering after processing. The presinteringtemperature is within a range of from 800° C. to 1200° C., especiallyfrom 950° C. to 1150° C., 1000-1100° C., or 1040-1080° C.

For facilitating the machining, the molded body according to theinvention is provided with relevant elements known to the skilled personthat are suitable for fixing the block within a CAD/CAM machine.

In a further embodiment of the process according to the invention, theporous sintered molded body obtained is further processed by formingprocesses.

The present invention also relates to a molded body obtainable by theprocess according to the invention. Advantageously, the layers aredimensionally stable in the porous sintering.

Example 1

Component 1 is obtained by intimately mixing 650 g of TZ-3YSB-C fromTOSOH, Japan (T₁˜1076° C.), with 350 g of TZ-PX-242A from TOSOH, Japan(T₃˜1038° C.), in a mixer from the company Bachofen, type DynaMix® CM200(or CM100, CM500, CM1000). For forming by means of an axial compressionmethod, component 1 is filled into the stamper by means of a fillingunit known to the skilled person. The layer is uniformly spread in themold by pulling away the filling unit. Typically, the layer thickness ishigher than the desired layer thickness of the final product because ofthe bulk density being lower than the compressed density, and depends onthe raw materials employed. Typically, the filled-in layer thickness ofthe powder is 2.318 times the corresponding layer thickness of thecompressed component and is 14.88 mm. Onto this layer, 22.29 g of amixture of 80% by weight component 1 and 20% by weight component 2,which consists of TZ-Yellow-SBC from TOSOH, Japan, is applied. After thesecond layer has been filled in, it is also smoothed and thereaftercovered by the third layer consisting of the intimately mixed components1 and 2 in a mixing ratio of 60% by weight to 40% by weight. Optionally,smoothing is again performed. Thereafter, the layered structure in themold is compacted under a pressure of 200-165 MPa to a density of 3.13g/cm³.

After the compressed molded body has been released, it is subjected tosintering until a temperature (T_(S)) of 1060° C. is reached. The moldedbody can be processed into multi-unit bridges by means of CAD/CAMmethods. The bridges obtained are dense sintered at temperatures of1530° C.

The corresponding colors of the individual layers can be seen from FIG.2A.

The sintered block is provided with a holding member. This is realizedby adhesive-bonding the block and holding member. The porous ceramiccomponent is placed by means of a centering device onto the holdingmember fixed in a base plate and provided with adhesive, and is thusfixed.

Example 2

The blocks shown in FIGS. 2B to 2D were prepared by analogy with Example1, the following mixing ratios of components 1 and 2 being employed:

Layer 1 Layer 2 Layer 3 Block Comp. 1 to Comp. 1 to Comp. 1 to accordingcomp. 2 comp. 2 comp. 2 to [% by weight] [% by weight] [% by weight]FIG. 2B 80 to 20 60 to 40 40 to 60 FIG. 2C 60 to 40 40 to 60 20 to 80FIG. 2D 40 to 60 20 to 80 0 to 100

The following Table illustrates the quantitative ranges in which thecomponent compatible with the first powdery ceramic material can bemixed with the first powdery ceramic material in order to achieve thedesired presintering temperature in the production process and to arriveat molded bodies that are according to the invention.

Proportion of Proportion of Proportion of TZ-3YSB-C TZ-PX-242ATZ-Yellow-SB-C in layer 1 in layer 1 in layer 2 Presintering [% by [% by[% by temperature Example weight] weight] weight] [° C.] 1 70 30 1001066 2 60 40 100 1066 3 50 50 100 1066

What is claimed is:
 1. A sintered ceramic molded body for dentalpurposes comprising a continuous color gradient and at least two layers,obtainable by a process, wherein a first powdery ceramic materialforming a layer is contacted with at least a second powdery ceramicmaterial forming at least a second layer; said first powdery ceramicmaterial has a presintering temperature T₁ that is higher than thepresintering temperature T₂ of said at least second powdery ceramicmaterial; said at least second powdery ceramic material exhibiting amore intense coloring than that of the first powdery ceramic material asevaluated with the CIE-Lab color system; the course of a curve ofshrinkage S₁ of said at least first powdery ceramic material differsfrom the course of a curve of shrinkage S₂ of said at least secondpowdery ceramic material, wherein curve of shrinkage S₁ is shiftedtowards higher temperatures as compared to curve of shrinkage S₂; andwherein the curve of shrinkage S₁ is shifted toward lower temperaturesby admixing the first powdery ceramic material with at least onecomponent that has a curve of shrinkage S₃ and a presinteringtemperature T₃ and being compatible with said first powdery ceramicmaterial wherein the at least one component material has a grain sizesmaller than the grain size of the first powdery ceramic material, toequalize the curves of shrinkage S₁ and S₂ in the region of apresintering temperature T_(S) with T_(S) being the presinteringtemperature at which the layers forming the molded body are subjected toa common temperature treatment to form a porous sintered molded body,the admixed materials having a curve of shrinkage that is between thecurves of shrinkage S₁ and S₃ in a region of T_(S); and the layers aresubjected to the common temperature treatment at T_(S) to causesintering that remains in a stage that has not proceeded to thetheoretical density, T_(S) being lower than T₁ and at least equal to T₃with the layers showing no distortions that are due to sinteringdistortions after completion of common temperature treatment; whereinthe at least one component material comprises zirconia, and whereincurves of shrinkage are measured under the same conditions.
 2. Themolded body according to claim 1, wherein T₁ is within a range of from900° C. to 1300° C.
 3. The molded body according to any claim 1, whereinT_(S) is within a range of from 850° C. to 1250° C.
 4. The molded bodyaccording to claim 1, wherein T₃ is within a range of from 850° C. to1200° C.
 5. The molded body according to claim 1, wherein the sinteredceramic molded body is further processed by forming processes.
 6. Themolded body according to claim 1, wherein the sintered ceramic moldedbody has a configuration suitable for a dental restoration.
 7. Themolded body of claim 1, wherein the at least one component is admixedwith the first powdery ceramic material in proportions within a range of25 to 50 wt %.
 8. The molded body of claim 1, wherein the grain size ofthe at least one component is at least 5% smaller than the grain size ofthe first powdery ceramic material.
 9. The molded body of claim 1,wherein the grain size of the first powdery ceramic material is from 80to 100 nm.
 10. The molded body of claim 1, wherein the continuous colorgradient has a range of L from 89.49-76.27.